The invention relates to motor control current sensor loss of assist mitigation for electric power steering (EPS).
EPS systems require the electric motor used to provide steering assist to be operated using a method of torque control. When using a Permanent Magnet Synchronous Machine (PMSM), Field Oriented Control (FOC) is utilized to allow the alternating current (AC) three-phase motor voltage and current signals to be transformed into a synchronously rotating reference frame, commonly referred to as the d/q axis reference frame. In a d/q axis reference frame, the motor voltages and currents become direct current (DC) quantities. The FOC torque control technique is commonly implemented either using feedforward methods of control or a closed loop current feedback control.
When a closed loop current feedback control is used, the ability of the system to regulate the torque is heavily dependent on the measured currents. However, current sensors, just like all sensors, are prone to failures. The most common forms of errors in current measurement are gain and offset errors. Offset errors can be particularly problematic, because depending on the magnitude of the error, the torque ripple caused by the offset error may become large enough to exceed requirements related to maximum steering effort.
A common method for mitigating loss of steering assist due to a current measurement fault is to transition from torque control utilizing a current regulator to achieve the desired motor current (and thus motor torque), to a torque control utilizing a static feedforward (inverse motor model) compensation when the fault is detected. However, a feedforward inverse motor model based torque control typically has much lower bandwidth as compared to a high bandwidth current control loop. The motor torque control loop in an electric power steering system is the actuator for the steering system, therefore should have a bandwidth several times higher than the outer steering assist control loop. The stability compensation for the steering assist control loop is designed in a manner suitable for the higher bandwidth of the torque control when the closed loop current control is active.
A stability compensation designed for the lower bandwidth feedforward inverse motor model based torque control used during a current sensor fault condition would be significantly different than the base stability compensation. This produces the undesirable result during a current sensor fault condition of the overall steering assist control loop being less stable in the faulted condition than in the nominal unfaulted condition.